Heretofore, apparatus and a method for determining dynamic friction in a threaded joint during an actual tightening operation was unknown. Accordingly, apparatus and a method utilizing a determined dynamic friction value for tightening threaded joints to a precise clamping load has not been available. The precise clamping load of a threaded fastener is extremely important in determining whether or not a joint, including the fastener, will fail in service. Consequently, threaded fasteners should be installed in a controlled manner, whereby the clamping load required to maintain the integrity of the joint is achieved.
One common technique for controlling the tightening of threaded fasteners is to use torque control apparatus by which a specific predetermined torque is applied in an attempt to attain a desired preload for particular thread and frictional conditions. Such an approach has the disadvantage that there may be variations in the torque/tension relationship from one tightening cycle to the next for the same assembly or same type of assembly due to different friction conditions from joint to joint, whereby clamping loads varying by as much as .+-.30% may be produced for a given applied torque.
Another known technique which is not dependent upon frictional conditions involves measuring the elongation of the fastener as the assembly is tightened. While this approach is capable of developing the accuracy required to achieve the desired clamping load, as a practical matter, in most cases direct measurement of elongation is either impossible or commercially unfeasible.
Yet another tightening technique which has been employed in the past in installing threaded fasteners is based on angle control. Given an estimate of fastener elongation required to achieve a desired clamping load, the threaded fastener is turned through a precise angle of tightening which will produce the necessary elongation. The disadvantage of this approach results from the difficulty in identifying the initiation point for the measurement of rotation of the fastener to produce the desired clamping load. U.S. Pat. Nos. 4,104,778 and 4,104,780 are directed to this technique and address the problem of identifying the point for initiating the measurement of rotation.
U.S. Pat. No. 3,982,419 is directed to an apparatus and method which involve tightening threaded fasteners into the yield region of the fasteners. Under such conditions, the disadvantages of the other techniques described above are avoided and the integrity of the assembly is greatly enhanced. There are, however, applications where the threaded fastener preferably is tightened to some point within its elastic range. For example, in the installation of certain high strength bolts, tightening to some clamping load below the elastic limit of the fastener will provide the desired tightened condition.